Poly-.alpha.-olefins such as polyethylene or polypropylene have excellent physical and mechanical properties and excellent processability, but since they are hydrophobic and chemically inactive, it is very difficult to dye fabricated articles of these polymers. In particular, crystalline polypropylene fibers have various features such that they are lightweight and strong and that they are good in heat retaining properties. Accordingly, at the initial period of the development thereof, these fibers were expected to have wide applications for clothes, etc. as "dreamlike fibers", but since they have a serious fault that they cannot be dyed by an ordinary dyeing method, the fibers have now been used in limited use fields for, e.g., uncolored waddings and carpet pile yarns.
Accordingly, an improvement in dyeability of polypropylene fibers has hitherto been keenly demanded, and various methods have been proposed.
For example, a method of melt spinning a mixture of polypropylene and a material which can be a dyeing site for anionic dyes is realistic and is being watched with keen interest in the point that the characteristics which polypropylene inherent possesses, such as easy spinnability and yarn strength, are not reduced and the point that various kinds of dyes common to nylon yarns and wool can be applied thereto.
In particular, a copolymer of ethylene and dialkylaminoalkyl acrylates comonomer represented by dimethylaminoethyl methacrylate, as described in Japanese Patent Publication No. 22523/67 is good in heat stability and can be very finely and uniformly dispersed in polypropylene by melt kneading and, hence, provides a dyeable poly-.alpha.-olefin composition having excellent spinnability.
Also, Japanese Patent Application (OPI) Nos. 66646/73 and 41345/84 show dyeability improving effects of the aforesaid composition by adding thereto various kinds of metal salts of organic carboxylic acids. (The term "OPI" as used herein means an "unexamined published application".)
On the other hand, in regard to the dyeing method, effects of various dyeing assistants and dyeing oils are shown in Japanese Patent Application (OPI) Nos. 154583/81, 154584/81, 133283/82, 149389/83, and 1786/84.
However, as the result of a number of trials for practical use made by the present inventors, it has been confirmed that in the case of dyeing fibers obtained by spinning various known dyeable polypropylene compositions containing a copolymer of ethylene and dialkylaminoalkyl acrylates comonomer as described above, using dyeing methods and equipments which are widely and normally used in the field of dyeing industry at present, the exhaustion speed of dye is low even by the above-described known improved dyeing methods and, hence, the dyeing time must be prolonged to an uneconomical extent.
For example, it has been found that when a tufted carpet made by using fibers obtained from the aforesaid dyeable polypropylene composition as pile yarns is subjected to wince dyeing at 95.degree. C. using a dyeing bath acidified by acetic acid, in order to obtain a satisfactory dyeing exhaustion rate, the dipping time must be prolonged to an extent of from about 1.5 times to about 2 times that in the case of dyeing a tufted carpet made by using 6-nylon yarns or wool spun yarns of the same type. If the dipping time is shorter than the above-specified range, the dyeing properties are deficient to dye only the surface of yarns, whereby the dyed carpet becomes insufficient in various fastnesses such as washing resistance, light resistance, and rubbing resistance.